Variations in human skin color are adaptive traits that correlate closely to geography and the sunís ultraviolet radiation, not race.

Credit: Angela Fast

Why do some people have light skin and others have dark skin? How do scientists explain the broad spectrum of human skin color around the globe?

Skin color, one of our most visible physical features, has long been used to divide people into racial categories. Anthropologist Nina Jablonski, Ph.D. theorizes that variations in human skin are adaptive traits that correlate closely to geography and the sunís ultraviolet radiation, not race.

"Over the course of evolution, human ancestors became bigger and more active as they moved into hot, open environments in search of food and water. In these places, one big challenge was keeping cool. The adaptation they made was to increase the number of sweat glands on their skin while at the same time reducing the amount of their body hair," explains Jablonski. With less hair, perspiration could evaporate more easily and cool the body more efficiently. "But this less-hairy skin was a problem because it was exposed to a very strong sun, especially in lands near the equator." Strong sun exposure damages the body. "The solution was to evolve skin that was permanently dark so as to protect against the sunís more damaging rays."